A. Field of the Invention
The present invention is generally related to electrostatic digitizers, and more particularly related to a method and apparatus for reducing noise in an electrostatic digitizer adapted to accept input via a pen or stylus or a user's fingertip.
B. Description of the Prior Art
Hand effect in an electrostatic digitizing panel causes a significant change in the pen or fingertip position recognized by the system when a user's hand (usually the one which holds the pen) gets close to the panel surface. This change of position is caused by the additional current injected to the panel by the user's hand being excited by the pen's electric field. Generally, the current flows in a closed circuit. Accordingly, the path of the current injected from a stylus to the panel is closed. Ideally, the path would close through the stylus capacitance to the earth ground. However, part of the return current flows back through the panel surface. The presence of the hand increases the amount of current following this path and distorts the original distribution of the current. This change may cause a significant error in the stylus position reported by the system.
Known to the art are both a method of increasing the thickness of the panel and a method of depositing a conductive indium-tin-oxide ("ITO") grid on the top surface of the panel. Increasing the thickness of the panel decreases the coupling between the hand resting on the panel and the resistive sensing layer deposited on the bottom side of the panel. Depositing an ITO grid on the top surface of the panel shields off some of the field emitted from the hand and provides a conductive pathway between the hand resting on the panel and ground. Both methods have limited effectiveness and introduce undesirable side effects such as increased panel thickness and visible grid lines on the panel surface.
A second problem encountered in the use of electrostatic digitizers is their sensitivity to electric and magnetic noise. Electric and magnetic noise may be emitted, for example, from the surface of a flat panel display on which the digitizer is mounted or by other system components such as a backlight converter. This noise may also distort the original distribution of the current and cause an error is the stylus or fingertip position reported by the system.
Accordingly, it would be advantageous to provide a method and an apparatus for reducing noise in an electrostatic digitizer caused by hand effect or sensitivity to electric or magnetic fields.